This invention relates to an image forming apparatus for developing an electrostatic image or a magnetic latent image in an electrophotographic method, an electrostatic printing method, or a magnetic recording method, and also to a method for forming an image where the image forming apparatus is employed.
When an image is to be formed by means of an electrophotographic system and if a two-component dry developing method is to be employed, a particulate toner is delivered from a developing apparatus and transferred via a carrier, an image carrier and, optionally, a transfer medium such as in intermediate transferring member, etc., to a recording material. Then, the toner on the recording material is subjected to heat and pressure so as to be fixed on the recording material. The toner in this case is enabled to adhere to each transferring medium through electrostatic force derived from the quantity of electric charge each toner particle has, van der Waals force, and liquid cross-linking force, i.e., adhesive force effected by water or moisture. The toner is transferred mainly through the mechanism that toner once adhered to one of the transferring medium is separated by the effect of external electric field and then permitted to adhere to a succeeding transferring medium. The toner is ultimately transferred over a recording medium such as paper and fixed as a pattern on the recording medium to form an image thereon. In order to efficiently transfer the toner to obtain a final image of high quality, it is desirable to control the adhesive force of toner to the transferring mediums.
As for the method of forming an image through the control of adhesive force of toner, there has been proposed a method of forming an image as shown in Japanese Laid-open Patent Publication (Kokai) No. 2002-328484 wherein the relationship among the adhesive force between the toner and an image carrier, an average particle size of toner, and the quantity of electrification is confined. In this case, there has been proposed a method of calculating the aforementioned adhesive force from the centrifugal force which is required to separate the toner from a transferring medium and which can be derived through the employment of a centrifugal separator.
Alternatively, Japanese Laid-open Patent Publication (Kokai) No. 2004-1011753, for example, describes a method of improving the transferring properties of toner wherein the toner is regulated to meet the condition of F/2σ>10 as the toner is subjected to centrifugal separation (wherein F is an average value in the distribution of toner adhesive force to be obtained from the measurement of adhesive force of toner after the tone is pressed onto the surface of an image carrier at a predetermine pressure; and σ is a standard deviation). In this method, it is intended that the distribution of toner adhesive force to be measured under specific conditions is greatly sharpened thereby to suppress non-uniformity of the transferring properties of toner and to make it possible to perform the transferring of toner efficiently and very precisely.
However, since this distribution of toner adhesive force is confined to an extremely narrow range, e.g. the a standard deviation σ is required to be not more than 0.3×10−8 as the average adhesive force is 6×10−8 N, the manufacture of toner becomes very difficult. Further, although it may be possible to enlarge the distribution of toner adhesive force to a certain extent by increasing the average adhesive force, if the toner adhesive force is increased too high, the transferring electric field required for the transfer of toner would become very high, thereby giving rise to risk of aerial discharge. Further, according to this measuring method, it is required to employ a step of pressing toner onto a recording material prior to the measurement of the adhesive force in order to reproduce the transferring pressure. According to this measuring method however, it is impossible to grasp the behavior of the toner which is weak in adhesive force, i.e., the toner which can be separated from an image carrier as the toner is subjected to weak transferring electric field immediately before the toner is introduced into the transferring nip. Moreover, according to this technique, there are possibilities that a small quantity of toner particle exhibiting an adhesive force which differs greatly from the average adhesive force may be included in the toner. Toner particle exhibiting considerably large adhesive force may become a cause for generating residual toner after the step of transferring the toner. On the other hand, toner particle exhibiting considerably small adhesive force may become a cause for generating the scattering of toner to a periphery of image. Because of these reasons, even with the employment of this technique, there are problems with regard to the transferring efficiency and quality of image.
In the cleaner-less process where a mechanism for recovering residual toner concurrent with the development of image, when the toner is caused to leave behind after the transferring step thereof, the succeeding electrification step and latent image-forming step are permitted to be undergone without the residual toner being removed, after which the residual toner in the non-imaging regions is recovered by a developing device concurrent with the development of new image regions. Therefore, if the quantity of residual toner after the transferring step is large, it may become causes for generating a defective image due to the incidents that the light source for forming a latent image may be obstructed, the recovery of toner by the developing device may become insufficient, and the generation of undesirable retransferring.
In the case of a color image forming apparatus of tandem structure, the toner that has been transferred to an intermediate transferring medium for example from an image carrier may happen to be reversely transferred to an image carrier of succeeding stage when the toner is subjected to a transferring electric field in the transferring region of the image carrier of succeeding stage and, at the same time, is press-contacted with the succeeding image carrier. Once this reversely transferred toner is recovered by the developing device in the cleaner-less process, the toner having the color of the developing station of the preceding stage is permitted to enter into the developing device of the succeeding stage, thereby making it impossible to perform the management of color if the toner entering the developing device of the succeeding stage is increased. The transferring efficiency frequently conflicts in nature with the reverse transferring efficiency. Therefore, in order to prevent such a situation where the color mixing due to the reverse transferring become too prominent to recover, it is required to adopt transferring conditions which make it possible to prevent the reverse transferring even at the sacrifice, to a certain extent, of the transferring performance.